This invention relates to wiring, and more particularly to small gauge coaxial wiring.
Certain demanding applications require miniaturized multi-wire cable assemblies. To avoid undesirably bulky cables when substantial numbers of conductors are required, very fine conductors are used. To limit electrical noise and interference, coaxial wires having shielding are used for the conductors. A dielectric sheath surrounds a central conductor, and electrically separates it from the conductive shielding. The desired miniaturization of cables and coaxial wires is limited by the dielectric capabilities of the sheath, with conventional insulating materials having inadequate dielectric resistance below certain levels of miniaturization.
Advanced dielectric materials have improved dielectric qualities that permit a thinner sheath to perform adequately. Such materials include PTFE (e.g. Teflon(copyright)). Further advancements include modified materials such as PTFE tape that has been stretched to generate a porous film with improved dielectric qualities. Wire manufacturing methods provide additional dielectric capabilities by helically wrapping such tape in an overlapping manner to form air spaces. However, such materials are expensive, and such techniques are slow, difficult to control precisely, and costly.
For large scale manufacturing, improved dielectric qualities of thermoplastic material has been provided by xe2x80x9cfoamingxe2x80x9d the plastic. This is done by dissolving gas in the molten plastic under pressure, then releasing the pressure to allow bubbles to form throughout the finished product. This is known to provide advantages of light weight, reduced material cost, lower capacitance (permitting faster signal speeds), and improved dielectric strength for a given thickness of material. However, these techniques have proven unsuitable for use in miniaturized applications. The size of a bubble limits the size of the product that may be usefully formed. When the bubble size approaches the wall thickness, unacceptable arcing is possible. Known foaming processes, particularly those that may have been used for coextrusion of wire insulation, generate bubbles larger than can be tolerated for small gauge wires used for coaxial components of miniaturized cable wire assemblies. With typical bubble sizes in the range of 0.004-0.008 inch, and a desired wall thickness of 2-10 times the normal bubble diameter, current foaming technologies are unsuitable for insulation of very small gauge wires with gauges less than 32.
The present invention overcomes the limitations of the prior art by providing a method of manufacturing wire from a conductor and a supply of thermoplastic insulating material. The method involves heating the insulating material, adding pressurized gas to the material, and extruding a sheath of the insulating material about the conductor to generate bubbles of the gas in the sheath. The method may operate by limiting the temperature of the material prior to extrusion, to increase viscosity to limit bubble expansion, and may provide a significant pressure drop on extrusion by limiting the gap between a wire-supplying guide and the extrusion die.